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What Is the Krebs Cycle?

The Krebs cycle occurs in all organisms that use oxygen to breathe.
Mitochondria hold enzymes necessary for the Krebs Cycle.
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  • Originally Written By: M. Dee Dubroff
  • Revised By: Bott
  • Edited By: Lucy Oppenheimer
  • Last Modified Date: 26 August 2014
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The Krebs cycle refers to a complex series of chemical reactions that produce carbon dioxide and Adenosine triphosphate (ATP), a compound rich in energy. The cycle occurs by essentially linking two carbon coenzyme with carbon compounds; the created compound then goes through a series of changes that produce energy. This cycle occurs in all cells that utilize oxygen as part of their respiration process; this includes those cells of creatures from the higher animal kingdom such as humans. Carbon dioxide is important for various reasons, the main one being that it stimulates breathing, while ATP provides cells with the energy required for the synthesis of proteins from amino acids and the replication of deoxyribonucleic acid (DNA); both are vital for energy supply and for life to continue. In short, the Krebs cycle constitutes the discovery of the major source of energy in all living organisms.

Discovery

The Krebs cycle, also known as the tricarboxylic acid cycle (TCA), was first recognized in 1937 by the man for whom it is named, German biochemist Hans Adolph Krebs. His highly detailed and extensive research in the field of cellular metabolism and other scientific endeavors gleaned him the Nobel Prize for Physiology or Medicine in 1953. Krebs's contributions to the fields of science and medicine are substantial; in addition to the citric acid cycle, Krebs also identified the urea cycle in 1932.

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Functions

Within the Krebs cycle, energy in the form of ATP is usually derived from the breakdown of glucose, although fats and proteins can also be utilized as energy sources. Since glucose can pass through cell membranes, it transports energy from one part of the body to another. The Krebs cycle affects all types of life and is, as such, the metabolic pathway within the cells. This pathway chemically converts carbohydrates, fats, and proteins into carbon dioxide, and converts water into serviceable energy.

The Krebs cycle is the second stage of aerobic respiration, the first being glycolysis and last being the electron transport chain; the cycle is a series of stages that every living cell must undergo in order to produce energy. The enzymes that cause each step of the process to occur are all located in the cell's "power plant"; in animals, this power plant is the mitochondria; in plants, it is the chloroplasts; and in microorganisms, it can be found in the cell membrane. The Krebs cycle is also known as the citric acid cycle, because citric acid is the very first product generated by this sequence of chemical conversions, and it is also regenerated at the end of the cycle.

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Discuss this Article

anon944748
Post 85

The Krebs cycle generates only two ATP molecules per glucose molecule. This is because, when glucose is broken down in glycolysis to produce two molecules of pyruvate, and each pyruvate molecule is carboxylated into an acetyl, each acetyl group goes through Krebs cycle only once, and produces one ATP molecule as a result. Therefore, the two acetyl groups together produce two ATP molecules-(one ATP molecule each).

anon340846
Post 84

How does rat poison affect the krebs cycle?

anon250435
Post 79

Can ATP supplements like PEAK ATP actually work?

anon201063
Post 76

I am a lay person, i.e., non medical. I have a 61 year old friend who ten years ago experienced nickel poisoning and, as a consequence, lost 25 kg in weight. Since the poisoning episode, his metabolism has been so rapid it precludes him from gaining weight no matter what or how much he eats.

I recently noticed on a poisoning web site while trying to research the matter, that nickel poisoning can interfere with the Kreb's cycle. Would that interference manifest itself in rapid metabolism and thus no weight gain? If so, is there anything that can reverse that interference and restore the balance of the Kreb's cycle?

I would appreciate any help but remember I am not a Wisegeek! --helpthegirl

anon143566
Post 71

I heard the Kreb´s cycle is enhanced when we do physical exercise. while exercising, how long does the Kreb´s cycle take from beginning to end?

anon135385
Post 69

And exciting new science and technology, 52 genes that influence and cause aging of the mitochondria in the cells of brain, heart and muscle have been identified. Also identified is natural products that can influence and slow these genes down. One experiences more energy, better sleep, decreased aches and pains, and even Alzheimer's and ADD/ADHD patients doing better. This is because the mitochondria are functioning better.

anon130492
Post 67

did krebs' research and discovery of the cycle lead any other scientist to discover any other medical things like diseases or vaccines or anything?

anon119629
Post 66

Kreb cycle actually forms a total of 38 electrons but two are lost in transportation making the net total 36.

@#54. I'm in Bio now and the only time glycerol is associated with any of this is during yeast fermentation where carbon dioxide and alcohol is produced. But this situation interests me a lot, so I'm definitely going to ask my teacher.

anon117035
Post 64

as the process is completed,the final products are:

3molecules of ATP from each NADH and H+ produced (10NADH + H+ x 3

ATP molecules=30 ATP molecules)

2 molecules ATP from each FADH

(2 FADH x 2ATP molecule= 4 ATP molecules)

1/2 oxygen + 2H+=water

total:34 ATP molecules

anon116495
Post 63

I understand the Krebs Cycle better when i read it online.

anon116169
Post 62

How do electrons enter and go through the Krebs Cycle to the electron transport chain?

anon111191
Post 61

can someone help me about the krebs cycle?

i can't understand how it is generated. i mean the stage?

anon110974
Post 60

can you please tell me that where the enzymes are located for tca or krebs cycle and electron transport chain in mitochondria?

anon109271
Post 59

Is there an easy way to memorize the cycle?

anon104927
Post 58

Could someone please tell me if eating too much refined sugars in your diet can mess up your energy levels and brain functions. It seems obvious that it would but I would like to know how it affects the Krebs cycle etc. Thanks.

anon101004
Post 57

Plants have mitochondria, and this is where their Krebs cycle happens. Chloroplasts are the location of photosynthesis. There are a few differences between plant and animal Krebs cycles, the chief being that plants produce the ATP directly, rather than via GTP.

If it seems like plants aren't important, try to do without oxygen or food. Yet many schools virtually ignore them as functioning organisms.

anon100871
Post 56

Glycogen converts to glucose, not glycol.

anon96800
Post 55

- anon73116: It's a great explanation. Thanks a lot, but I have one major question that digs my head!

You mentioned GDP(guanosine DiPhosphate)+PO4(Phosphate)to form GTP(guanosine TRIPhosphate)perfect!

Then GTP hands off the phosphate; ADP bonds with it to form ATP. This is what i've understood

My question is why is there GDP or GTP, in order to accomplish ADP to ATP?

i have never heard guan di/triphosphate before

so I was lucky to learn it, but the why i don't understand.

What's the function of GDP, if only to carry phosphate to ADP? Why can't ADP directly bond to phosphate? You know, I have all sorts of questions related with GDP and GTP. I would be glad if you explain clearly.

Thanks a lot.

trufeather77
Post 54

Question: I remember, from Bio class many moons ago, that the Krebs cycle starts out with converting glycogen to glycol? Something to do with glycol, at least, which is an alcohol.

I found this interesting because of a phenomenon in my family: only two family members have had a problem with alcohol, and both of those were "preemies" -- babies whose birth was immature. The younger of the two was born in 1954, and by then they were still feeding preemies like hummingbirds, with a kind of sugar water. If this breaks down in the body like alcohol, then as an adult, these "preemies" would find alcohol comforting, like "mother's milk."

A friend told me that I'm wrong, that glycol has nothing to do with the Krebs cycle, but I distinctly remember it, and remember being shocked that glycol (or anything ending in "ol") is a kind of alcohol. Anyone able to help?

(I've tried looking it up on a variety of sites, but some descriptions are so technical, I don't know what I'm reading.)

anon95181
Post 53

Where do the electrons go?

anon79972
Post 48

How does the electron transport chain activate during the krebs cycle affect the hydrogen ion concentration in a mitochondrion?

anon78946
Post 47

what two major biochemical reactions occur in the Krebs cycle?

anon78520
Post 45

thank for your information.

anon73116
Post 43

The molecule that enters the krebs cycle is a form of pyruvate that has been modified known as simply as Acetyl-CoA. This molecule is made up of Acetate and and a Coenzyme simply known as coenzyme A. Acetyl-CoA is received by oxaloacetate (the molecule that is regenerated) to form citrate.

Water is then added and removed at the same time to form an isomer of citrate known as Isocitrate. Then what happens is NAD+ is reduced to NADH and a H+ is also released along with a CO2 to form alpha ketoglutarate. Then another CO2 is released along with an H+. Again an NAD+ is reduced and CoEnzyme A bonded to a sulfhydryl group is added to the molecule forming Succinyl CoA. The Coenzyme A and the sulfhydryl group is removed and an inorganic phosphate is added to the molecule. Here is where the 1 ATP is made.

The inorganic phosphate is added to GDP (guanine diphosphate) to form GTP. The GTP then "hands of" the phosphate group to an ADP through substrate level phosphorylation to form the ATP.

This process makes the next molecule in the cycle: succinate. Then an FAD+ is reduced to FADH2 forming fumarate. Water is added to fumarate to form malate, and finally one last NAD+ is reduced and one H+ is released forming oxaloacetate! The cycle can now start again.

While each pyruvate yields only 1 ATP per turn of the cycle, there are two pyruvates per molecule of glucose, so in reality, two molecules of ATP are formed from the kreb cycle even though is only appears as one! Hope this helped!

anon72259
Post 42

what two high-energy molecules are produced in the Krebs cycle?

anon69950
Post 41

The discussion generates an amalgam of inputs, some from very biochemically enlightened minds, some from profound laymen (biochemically).

anon69351
Post 40

anon11925, anon15652 is wrong. The entire process of respiration produces 36 to 38 ATP, however the Krebs Cycle on produces two.

anon67316
Post 38

How many molecules of O2 in the kreb cycle and how many molecules of co2 are produced?

anon67130
Post 37

I was asked to answer this question what will be your own answer.

1. What is the number of ATP formed in Kreb cycle?

2. The net ATP formed from one molecule of glucose at the end of kreb cycle is?

anon64467
Post 36

please can anyone explain (as simply as possible) the link/pathway between the following: hypertonic (or shortened) muscle causing muscle ischemia which then impacts on the Krebs cycle (in some way) and then causes hyperglycemia?

anon62205
Post 35

Could you please tell me how the Krebs Cycle affects someone hours after eating a meal?

anon61894
Post 34

why is krebs cycle so hard to understand?

anon58477
Post 31

for each pyruvate that enters the krebs cycle once the cycle has been completed, you gain 1 ATP!

anon58042
Post 30

Questions about the Kreb's cycle:

1. What molecule receives the hydrogen atoms from the carbon molecules?

2. Where do the oxygen atoms from the carbon atoms end up?

3. Where does the carbon dioxide end up? How does it exit the body?

thank you for your time. =)

anon54315
Post 26

The krebs cycle makes one molecule of ATP pre cycle (per pyruvate) but if your talking about per glucose, it's 2 molecules of ATP, 6 Reduced NAD, 2 reduced FAD and 4 Carbon dioxide. ATP is made by substrate level phosphorylation (addition of a phosphate molecule).

ATP is short for Adenosine Triphosphate. When it is hydrolyzed a phosphate group is released. This is the breaking of a bond and when bonds break, energy is released.

Hope this answered some questions.

anon51936
Post 24

how many molicules of ATP is produced in the whole krebs cycle?

anon51430
Post 22

I was doing a report on Sir Hans Krebs and I didn't know what the Krebs cycle was. so this was really helpful. Thanks.

anon48230
Post 21

From experience in my practice and plenty of research by Dr. Weissman, a product called Citrokehl from Sanum helps assist the generation of the Krebs Cycle, an isopathic remedy containing citric acid. That may assist the chronic fatigue.

anon46900
Post 20
anon46502
Post 19

Wow! thanks

anon43672
Post 18

Wonderful answer. thanks.

anon42011
Post 17

Is it necessary to control the krebs cycle? What numbers are normal and what is atp? What do we do to eat/drink/do in children? Is it linked to autism?

anon39785
Post 16

why is it that the average person has enough stored fat to fuel a 750 mile run, but even well trained athletes fatigue after only about 25 miles?

anon38684
Post 15

Dear Editor: Could you explain any potential connection between the Krebs "Citric Acid" cycle and the diet where all you consume for days is a combo of: water, lemon juice, cayenne pepper and dark amber maple syrup? (a.k.a. Stanley Burroughs "Lemonade" or "Master Cleanse")

I tried this "lemonade" for 4 days without food - I lost cravings, had energy and focus, slept better, lowered blood pressure, etc..

anon37373
Post 14

A 72 year old gentleman suffers from chronic fatigue. I too, at the age of 63 suffered. However Patrick Holford's "Low GL Diet Bible", chapter 16 may be of use to the 72 year old as it has been for me. I have to pay for my own supplements and Chromium, but it has been really worth it. I work 12 hour shifts and feel both mentally and physically very much stronger than I have done for years. I started taking Holford's advice after a period on Statins, which I felt were killing me. My doctors have no faith in them either (work and private). Give Holfords advice a go. At the very least read into the side affects of statins; in detail, and about the way they inhibit cholesterol and importantly Co-enzyme Q10 which is vital to us all. You might start to see the reason for both Holford's and my doctors' scepticism about the so- called "Virtues of Statins". My view is, they were killing me, albeit slowly. However, you may not be on statins. But Holford may help.

anon35855
Post 13

This is an excellent refresher for someone that has studied anatomy and physiology in the past.

anon31191
Post 12

Finally something I can understand!

anon28322
Post 10

Exactly what is the relationship of photosynthesis to respiration and what effect does it have on the world?

anon28011
Post 9

Could u please give a detailed or point by point importance of the krebs cycle?

anon23054
Post 7

why is the krebs cycle so hard to understand?

anon22774
Post 6

You say nothing about how to assist the generation of the Krebs Cycle. I understand that Magnesium and Malic acid combo can do it, which I am trying and it seems to work! I am 72 and have been suffering from chronic fatigue.

anon22663
Post 5

Does the Krebs cycle release energy that is used to produce ATP?

decatlon14
Post 4

great article about krebs c. could you also provide the chemical reactions with some diagrams thank you again. Great web-site.

Moderator's reply: Glad the article helped! Google "krebs cycle diagrams," and you should find some diagrams that should help as well.

anon15652
Post 2

The krebs cycle theoretically produces 36 to 38 total atp. The cycle actually will produce less than that theoretical yield in reality, but 36 to 38 atp is what is supposed to be generated.

anon11925
Post 1

How many ATPs does Kreb Cycle generate and how?

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